Integrated magnetic levitation and rotation system

ABSTRACT

A rotary motor and a rotary magnetic bearing are integrated in a compact assembly that is contact-less. A stator assembly surrounds a ferromagnetic rotor with an annular air gap which can accommodate a cylindrical wall, e.g. of a chamber for semiconductor wafer processing. The stator assembly has a permanent magnet or magnets sandwiched between vertically spaced magnetic stator plates with plural pole segments. The rotor is preferably a ring of a magnetic stainless steel with complementary pole teeth. The stator assembly (i) levitates and passively centers the rotor along a vertical axis and against tilt about either horizontal axis, (ii) provides a radial position bias for the rotor, and (iii) establishes a motor flux field at the rotor poles. Polyphase coils wound on the stator plates produce a rotating flux field that drives the rotor as a synchronous homopolar motor. A rotor without pole teeth allows operation with an asynchronous inductive drive. A controller energizes control coils wound on each stator pole segment in response to a sensed physical position of the rotor. The control coils provide active radial position control and can actively damp tip and tilt oscillations that may overcome the passive centering.

This application is a continuation of application No. 08/548,692 filedOct. 26, 1995, now U.S. Pat. No. 5,818,137.

BACKGROUND OF THE INVENTION

This invention relates in general to magnetic bearings andelectromagnetic motors. More specifically, it relates to a process andapparatus providing an integrated, highly compact, integrated rotarymagnetic bearing and motor particularly one useful in semiconductorwafer processing.

In semiconductor fabrication, fragile wafers of silicon or othersemiconducting materials must be processed in a controlled, super-cleanatmosphere, whether that atmosphere is a vacuum, an inert gas, or aprocess gas. Microscopic contaminates in the atmosphere are a severeproblem since they can deposit on the wafer, either directly or as aresult of a gaseous processing of the wafer. Microscopic particles on awafer contaminate it; semiconductor devices made from the contaminatedportion of the wafer will be defective. Cleanliness is thereforedirectly related to yield, which in turn affects final product cost.Modern processing techniques include using multiple chambers connectedby vacuum or inert gas-filled transports. These chambers are specializedfor certain process steps. Processing usually involves hostileatmospheres of highly corrosive gases and high temperatures.

One processing step is an annealing of the wafer following doping by ionimplantation. The implantation produces strains in the crystal structurewhich, if not relieved, cause unwanted variations in the resistivity ofthe ion doped silicon. The annealing is important to relieve thesestresses. Modern fabrication has turned to rapid thermal processing(RTP) for annealing. RTP involves the use of a radiant heat source(e.g., a lamp) mounted in a vacuum chamber over the wafer. The lampquickly brings the wafer to a suitably high temperature for theprocessing. For RTP, it is necessary to have the region over and underthe wafer unobstructed.

Uniformity of processing over the entire wafer is important because thewafer is eventually subdivided into multiple discrete devices, e.g.,microprocessor or memory chips, that each must have generally the sameknown characteristics. Uniformity is also important where it is desiredto use a relatively large surface area for a single device, e.g.,fabrication of an entire computer on a single chip. In short, uniformityalso influences yield. To produce uniformity, it is conventional torotate the wafer about a vertical or z-axis perpendicular to, andcentered on, the wafer as it is being processed. Rotation is also usedfor other wafer processing such as chemical vapor deposition, heattreatment, doping by ion implantation, and doping by other techniques.

Heretofore conventional and RTP wafer processing equipment has usedmechanical contact bearings to support a rotatable platform which inturn supports a wafer carrier. Mechanical contact bearings, however,even the custom designed, highly expensive bearings now in use, are thesource of many problems. First, because they make moving contact, theywear. This wear is the source of particle contamination. Second, due tothe wear and operation in a difficult environment (an ultrahigh vacuum)or hostile environment, (e.g., a corrosive or high temperatureatmosphere), there are severe constraints on lubrication, the bearingsfail unpredictably, and they typically fail in a comparatively shorttime. (Lubricants evaporate particularly when exposed to a high vacuum.Seals and choice of lubricants can help to control the problem, butbearing lubrication remains a source of contaminants.)

Bearing failure is the cause of significant production down-time. Italso raises manufacturing costs due to the direct cost of new bearingsand often the loss of a wafer being processed, which may itself have avalue in the range of $50,000. Even without bearing failure, wafersbreak. Besides the loss of the wafer, breakage also produces asignificant loss of production time since any wafer break produces waferfragments and particles which must be meticulously cleaned out of thebearings before the chamber can be used again. When operating,mechanical bearings also produce vibration (they are noisy) and theytransmit, and can be worn or damaged by, external mechanical shock andvibration. Mechanical bearings also suffer from stiction and "play"which introduce error in position control which adversely affect qualityand yield. These and other problems also impose a practical upper limiton the size of the wafers that are processed and the speed of rotationof the wafer. At present wafers with diameter of about 200 mm are thelargest which can be processed reliably. Rotational speeds are typicallyno greater than 90 rpm for these large wafers.

Magnetic handling has been considered for use in wafer processing. Forexample, M. Ota et al. in "Development Of Mag-Lev Polar Coordinate RobotOperation in Ultra High Vacuum", Magnetic Suspension II, pp. 351-359(1991) describe a polar coordinate robot for operation in a vacuum usingmagnetic bearings. Magnetic bearings for contactless suspension andlinear movement of wafers is also the subject of papers such as S.Moriyama et al., "Development of Magnetically Suspended Linear PulseMotor for Contactless Direct Drive in Vacuum Chamber Robot", KyushuInstitute of Technology, Transaction of the Institute of ElectricalEngineers of Japan, Vol. 115-D, No. 3, March 1995, pp. 311-318. Whilethe advantages of magnetic suspensions to avoid of contaminationproblems of mechanical bearings are clear cut, this kind of research hasbeen limited to wafer transport, particularly linear wafer transportbetween process chambers, not for use in a chamber itself to rotate thewafer during processing.

The general use of magnetic bearings to support a rotary motion areknown. The are typically used where it is important to eliminatefrictional contact totally. In a common arrangement, a pair of axiallyspaced magnetic bearings support a rotary shaft which is driven by anaxially interposed electric motor. The rotor of the motor is typicallysecured to the shaft, which is the output shaft of the motor. Magneticbearings, both radial and axial, are also used in devices as diverse asgyroscopes, flywheels, gas turbines, and electrical measuringinstruments.

While the advantage of frictionless operation and operation at adistance to control movement in a sealed chamber or the like are clear,magnetic bearings are not widely used because they are bulky andexpensive. Cost is driven up in part by the need for position sensorsand active feedback control circuitry to suspend and center the bearingin a preselected spatial location and orientation, typically involvingcontrol over six degrees of freedom--linear motion along three mutuallyorthogonal (x,y, and z) axes and rotation about each of these axes.

It is also known to reduce the cost of active control by passive controlof at least one degree of freedom. In a common form, this passivecontrol uses magnetic repulsion between a pair of permanent magnetsarrayed along one axis. LC tuned circuits are also known to vary amagnetic field generating current in a coil of inductance value L in amanner which provides a passive control. The coil inductance L and acapacitor with a value C are series coupled. An A.C. excitationfrequency is set just above the LC resonant frequency. Because theinductance of the coil is very sensitive to the air gap between the coiland the rotor, changes in the gap automatically produce changes in theimpedance of the LC circuit, which in turn adjusts the current flow toinduce a centering.

Regardless of whether passive controls are used, if a driven rotatingmember is magnetically supported, it must also be driven. With a directmechanical drive, there is the problem of how to transmit the rotarypower into a chamber without a frictional contact that is open to thechamber (a source of contaminants) while maintaining a controlledatmosphere within the chamber (e.g. a rotating shaft held in a seal). Amagnetic drive can overcome this problem, but it introduces the bulk andcost of this type of drive, as well as further problems such as theinteraction of an AC flux of the drive with a DC flux of the suspension.For sealed chamber processing, the AC drive flux produces furthercomplications. The AC flux produces eddy current losses in the chamberwall as well as losses in the stator and rotor. Second, the air gap thataccommodates the chamber wall constitutes a significant source ofreluctance in the magnetic circuit. Third, the AC flux acting on therotor competes with the substantial saturation of the rotor by the DCsuspension flux. In short, there are significant design considerationthat lead away from integrating a rotary magnetic drive with a magneticsuspension.

To date, no known system uses the frictionless suspension of magneticbearings in wafer processing, such as RTP in combination with a rotaryelectromagnetic drive of the wafer. More generally, no compact,cost-effective arrangement has been devised which integrates thelevitation and frictionless operation of a magnetic bearing with arotary electromagnetic motor drive.

It is therefore a principal object of this invention to provide aprocess and apparatus for a combined, integral rotary magnetic bearingand rotary drive that uses no physical contact between moving partsduring its operation.

A further principal object of this invention is to provide such anintegrated magnetic bearing and drive process and apparatus which arehighly compact, both physically and in the active control needed toestablish and maintain the bearing gap.

Another object is to provide an integrated magnetic bearing and drivewith the foregoing advantages which can operate reliably and with a longlife in a vacuum, in a corrosive atmosphere, or at high temperaturessuch as those encountered in wafer processing, particularly RTPprocessing.

A further object is to provide an integrated magnetic bearing and drivewith the foregoing advantages which can process very large diameter(e.g., >300 mm diameter) wafers and rotate them at over speed rangesfrom 50 to 1200 rpm.

A still further object is to provide a magnetic bearing and drive forwafer processing which reduces the particulate contaminants in thechamber and production down-time due to particle contamination ormechanical bearing failure.

Another object is to provide an integrated magnetic bearing and drivewhich allows a high degree of precision in positioning, produces novibration, and isolates the wafer from external vibration and shock.

Still another object is to provide these advantages with a competitivecost of manufacture.

SUMMARY OF THE INVENTION

A compact, integrated magnetic levitation and rotational drive utilizesa magnetic rotor and a surrounding, non-contacting stator assembly. Thestator assembly has a permanent magnet or other source of DC magneticflux sandwiched between mutually vertically spaced pole pieces,preferably plates, of a ferromagnetic material with a low reluctance.The plates are preferably divided into plural equangularly spaced polesegments, e.g. four segments, that closely surround the rotor with anannular air gap. The rotor is preferably a ring with a complementary setof poles, e.g., eight doubly salient sets of pole teeth, formed on itsouter periphery and positioned vertically between the stator plates.This arrangement uses the flux of the permanent magnet to (i) passivelycenter the rotor along a vertical z-axis, (ii) radially bias the x-yposition of the rotor within the stator assembly in a horizontal, x-yplane, and (iii) provide a flux field in the rotor poles which caninteract with a rotating field established by suitably energizedpolyphase motor windings on the stator assembly.

Each stator pole segment carries active position control coils that arepreferably wound between arcuate slots in the body of each stator plate.The radial, vertical, and angular positions of the rotor are sensed witheddy current and Hall effect or equivalent sensors positioned proximatethe rotor. Closed-loop feedback control circuits of a controller producecurrents that energize the control coils to produce a magnetic flux thatadds or subtracts from the DC flux field produced by the permanentmagnet. The currents adjust the flux field strength to maintain therotor in a radially centered x-y position. It can also operate tocontrol oscillations of the rotor out of the x-y plane which the passivereluctance centering of the permanent magnet flux field cannot control,at least at high rotational speeds. Ideally out-of-plane vibrations arecontrolled solely by passive means such as inter connection of coils andselection of materials such that these vibrations cause eddy currents tobe generated which damp the oscillations.

An alternative form of the invention use a toothless rotor to provide anasynchronous induction motor. Alternative passive radial positioncontrols use capacitors in series with the active control coils and ACexcitation of the LC circuit set just above its resonant frequency.Changes in the stator-to-rotor air gap couple to the coil to producechanges in the inductance L of the coil, which causes the current tovary in a manner that drives the rotor toward the desired centeredposition. Position sensing can also be estimated. A small signal issuperposed on the control coil current at a high carrier frequency tomonitor the air gap in the magnetic circuit. Another position estimationmethod measures the control coil current directly. A microprocessoranalyzes the sensed coil current to yield radial position information.

These and other features and objects of the present invention will bemore fully understood from the following detailed description whichshould be read in light of the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of an integrated magnetic levitation and rotationassembly in accord with the present invention along the line 1--1 inFIG. 1;

FIG. 2 is a view in vertical section of the integrated magneticlevitation and rotation assembly according to the present invention, asillustrated in FIG. 1, operating in conjunction with a chamber, shown inphantom, for RTP of a semiconductor wafer;

FIG. 3 is a detailed view in vertical section of the stator assembly androtor shown in FIGS. 1 and 2 and adjacent portions of the chamber wafercarrier and a wafer; and

FIG. 4 is a perspective view of the rotor shown in FIGS. 1-3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1-4 show an integrated magnetic motoring and suspension system(IMMSS) 10 according to the present invention adapted for use inconjunction with a sealed chamber 12 for processing silicon wafers 14into multiple discrete semiconductor devices such as microprocessors ormemory chips. The IMMSS 10 features a stator assembly 16 that surroundsa rotor 18 with a radial spacing, a thin annular air gap 20. The rotor18 supports, and is replaceably coupled to, a wafer carrier 22, which inturn carries the wafer 14 on its upper surface. The rotation of therotor about a vertical z-axis 24 rotates the carrier 12 and wafer 14about the same z-axis, which during operation is generally coincidentwith a common centerline of the rotor, carrier, and wafer. Rotation ofthese elements is preferably constrained to lie a single horizontal x,yplane where the x-axis 26, y-axis 28 and z-axis 24 are mutuallyorthogonal.

The invention will be described herein with reference to its presentprincipal application, the successive processing of multiple wafers 14in the chamber 12, and in particular, a rapid thermal processing (RTP)of the wafer 14 to relieve crystal lattice strain produced by dopant ionimplantation. However it will be understood that the invention is usefulin other semiconductor processing steps where it is necessary ordesirable to rotate a wafer in a controlled or hostile environment, ormore broadly, whenever it is required to rotate and suspend a body,particularly where the body is in a controlled atmosphere.

The RTP chamber 12 has a lower housing 12a and an upper housing cover12b that mate in a seal sufficient to maintain a vacuum within thechamber (e.g. 0.001 Torr), but also separate to allow direct access tothe interior of the chamber as necessary. Screws 30 secure that statorassembly 16 to a wall 12c of the chamber housing 12a. The rotor 18 isreceived in an annular well 12d formed by a cylindrical chamber wall12e, a bottom flange 12f, and a cylindrical wall 12g capped by acircular bottom plate 12h which underlies the wafer carrier 22 with aclearance 32.

This geometry allows a simple drop-in insertion and removal of the rotor18 from the chamber, which, because the rotor is ferromagnetic, can beperformed magnetically using robotics, or manually. If there is a waferbreak, removal of the rotor 18, carrier 22, and the remains of the wafer14 leaves only smooth surfaces that can be readily cleaned of waferparticles in a small fraction of the time required to clean conventionalmechanical bearing assemblies. This geometry also leaves access to thewafer, wafer carrier, and the wafer unobstructed from above and below.This is important in establishing a well defined thermal gradient acrossthe wafer using a lamp 34, or equivalent source of radiant thermalenergy, under the control of one or more optical pyrometers 35, or thelike, mounted in the housing portion 12h. This geometry is also highlycompatible with a fully robotic, fully integrated magnetic handling andtransport system which positions the wafer and/or its carrier andtransports it or them to and from other processing stations.

As well be discussed below, a rotor-levitating magnetic flux flowingfrom an upper pole piece of the stator assembly, through the rotor to alower pole piece as elements of a common magnetic circuit is central tothe operation of the IMMSS. Therefore, the wall 12e in the annular gap20 must be thin, nonmagnetic and not introduce significant eddy currentlosses. In the presently preferred embodiment, with a rotor diameter oftwelve inches (307 mm), the wall 12e is preferably 0.035 inch thick andformed of type 316 stainless steel. The other chamber walls are alsopreferably formed of the same material.

The stator assembly 16 has a pair of vertically spaced pole pieces 36,36 in the form of generally flat plates that sandwich a permanent magnet38, which, as shown, is preferably four identical, equiangularly-spacedpermanent magnets 38a, each associated with, and angularly centered on,a pole segment 36a. The four magnets 38a are mounted generally at theouter periphery of the plates 36, 36. They are capable of producing astrong DC magnetic flux field at the rotor despite the presence of airgaps. The DC flux field is strong enough to levitate the rotor andstiffly center it vertically. Magnets of samarium cobalt, various gradesof neodymium boron iron, and ceramics are suitable. Ceramic ferritemagnets are preferred for low cost. Samarium cobalt may be used wherethe operating temperature range and demagnetization are concerns.Neodymium offers the most magnetic flux for a given size.

The plates 36,36 are formed of a magnetic material with a high magneticpermeability and a corresponding low reluctance. Low carbon steel ispreferred due to its low cost. The DC magnetic flux produced by themagnetic flow from the upper pole plate, across the gap 22--which in theillustrated embodiment includes the chamber wall 12e--to the rotor 18which closes this magnetic circuit. In the preferred form shown, thehigh reluctance air gaps between the rotor and stator plates areminimized by forming two radially outward extending flanges 18a and 18bon the rotor. They are mutually spaced in the vertical direction so thatthey each can align generally with one of the plates 36. The rotor isalso formed of a ferromagnetic material with a high magneticpermeability and corresponding low reluctance. Preferably the rotor is aring of low carbon steel with a nickel plating. Alternatively, siliconsteel can be used for lower reluctance or 17-4PH stainless steel forhigher corrosion resistance.

The DC flux set up by the permanent magnet 38 and its pole pieces 36, 36induces a magnetic flux field in the rotor of opposite polarity, much asa permanent magnet magnetizes a nearby soft iron member with an oppositepolarity, causing them to be drawn toward one another (and therebyminimizing the intervening air gap, which reduces the reluctance of themagnetic circuit) Similarly the stator assembly 16 and rotor 18 areconstructed to utilize this tendency to minimize the circuit reluctanceto close the air gaps 22a and 22b between the upper and lower plates 36and the flanges 18a and 18b respectively. The magnetic forces in theseair gaps are analogous to a pair opposed mechanical springs that combineproduce a leveling effect as well as the levitation of the rotor. Therotor flanges each have eight poles 18c sized and spaced to match thefour stator poles segment 36a, as shown in FIG. 1. Because there are twoflanges 18a and 18b, poles 18c are formed in a matching pattern on bothflanges to produce a like saliency.

The DC flux of the permanent magnet also produces an attractive magneticforce between the poles of the stator and the rotor. This attractiveforce is radially directed and establishes a radial bias. However, it isunstable and tends to pull the rotor radially to one side. Additionalradial positions controls are therefore necessary, as will be discussedbelow. It is also important that the DC magnetic flux of the permanentmagnet 38, by magnetizing the rotor and its pole teeth, also induces arotor flux field. The rotor field interacts with a rotatingelectromagnetic field to develop a torque about the z-axis, and therebydrive the rotor.

The stator assembly includes two sets of coils, a drive coil 40 andmultiple active control coils 42 each associated with one plate of oneof the pole segments 36a. The drive coil 40 is preferably a polyphasewinding with end windings held in a series of notches 44 formed on theinner edges of the stators plates 36,36. The drive winding 40 isenergized by a conventional multiphase AC drive current to develop arotating electromagnetic field that interacts with the DC magnetic fluxfield of the rotor to cause it to rotate.

The control coils 42 are eight in number, each wound on one of the upperand lower plates 36,36 through body-centered arcuate slots 46 that arealso angularly centered on radial slots 48 that separate and define thestator poles segments 36a. The control coils 42 each carry a controlcurrent that produces a magnetic flux. Depending on the direction of thecurrent, this flux adds or subtracts from (modulates) the DC magneticfield acting on the rotor. Coordinated adjustments in these currentsstabilize the radial position and orientation of the rotor, both staticand dynamic. The radially centered rotor position leaves a clearance onboth sides of the rotor so that there is no frictional contact with thechamber walls as it rotates. The aforementioned levitation raises therotor from the flange 12f at the bottom of the well 12d.

Because there are coils on both pole pieces forming each pole segment,adjustments in the coil currents in vertically paired coils can alsocontrol the tip or tilt of the rotor out of a desired x-y plane. Notethat the passive reluctance centering produced by the DC magnetic fieldalso passively controls tip and tilt, but if this controlling field isweak, or if the rotational speed is sufficiently high, there is atendency for the rotor nevertheless to oscillate out of the desired x-yplane, or to reach a tilted equilibrium position. The control coils 42,energized in a "push-pull" manner in a vertically spaced pairs as wellas on diametrically opposite sides of the rotor maximize control oversuch oscillations, or any deviations of the rotor from the preselectedvertical position and preselected horizontal orientation.

In the presently preferred form shown for high speed operation withactive position control, four radial positions sensors 48, four verticalposition sensors 50, and three commutation sensors 52, are mountedproximate the rotor in the side chamber wall 12g, bottom flange 12f, andon the inner periphery of the pole pieces 36, 36 respectively. Theposition sensors are standard eddy current sensors, or knownequivalents. They produce signals which correspond to the physicalposition, radial and vertical, of the rotor. The commutation sensors 52are standard Hall effect sensors; they sense the angular position of therotor as determined by the presence or absence of a tooth 18c. Angularposition is used for conventional electronic commutation of the drivewindings 40. The sensors 52 are preferably mounted in alternate notches44 in the pole plate 36,, over the drive winding 40 in that notch.Radial and vertical position information is input to a controller 54which produces output currents to each of the position control coils 42.For clarity, the lines connecting the sensors and coils to thecontroller 54 are shown in FIG. 1 only with respect to one coil 42 andtwo sensors 48 and 50. However, it will be understood that outputsignals from all sensors 48 and 50 are supplied as inputs to thecontroller 54.

The controller 54 is preferably Texas Instruments model DSP C30controller, by or any of a variety of equivalent devices well known tothose skilled in the art. The magnitude and direction of the currentapplied to the associated coils 42 produces a modulation of the fluxfield at the rotor which (i) actively radially centers the rotor.Because three degrees of freedom--position along the z-axis and rotationabout x and y axes--are controlled passively, the cost and bulk of theactive control circuiting is reduced substantially as compared to asystem of total active control in all degrees of freedom. The controlcurrent can also actively control tip and tilt out of a preselected x-yplane, and in particular damp oscillations which appear at highrotational speeds and which the passive vertical centering of the DCmagnetic flux is not able to control adequately.

By way of illustration, but not of limitation, for a rotor with a twelveinch (307 mm) diameter with eight doubly salient poles made of stainlesssteel in the form shown in FIG. 4, the rotor has a weight of about 5.5pounds (2.5 kg). A quartz ring carrier 22 and wafer 14 have a weight of2.0 pounds for a total weight of 7.5 pounds (3.4 kg) to be levitated,centered and rotated. A magnetic gap 22 of 0.080 inch, a housing wall12e thickness of 0.025 inch (0.6 mm) of 316 stainless steel, and thepole count and type described and shown above, the permanent magnet 38develops a field of 3,500 gauss for the levitation, reluctancecentering, radial housing and rotor field magnetization. The rotor issuspended with a vertical stiffness of about 200 lbs./inch. The controlcoil 42 has a maximum power rating of 18 watts per axis. The runningpower is about 1 watt per axis. Operated as a synchronous homopolarmotor, this IMSS develops a maximum torque of 3.0 Nm with a maximumrotational speed of 1200 rpm, taking about 6.0 seconds to spin up tooperating speed from a standstill. Maximum motor power at spin up is 200watts, and the power at normal operating speed is estimated at about 100watts, which in part reflects eddy current losses.

While the preferred embodiment uses position sensors and a positioncontroller, the sensors and their associated electronics are a majorcost component of the IMMSS 10. An alternative embodiment reduces thesecosts with passive radial position control. A capacitor 56 (shown inphantom) is connected in series with each radial control coil 42. Afixed amplitude, fixed frequency A.C. voltage excites the circuit. Thecircuit has a resonance at a frequency f_(n) =√1/LC, where L is theinductance of each coil 42 and C is the capacitance of the associatedcapacitor 56. At the resonant frequency f_(n), the impedance of the LCcircuit is purely ohmic, and the current in the coils is thereforelarge. By operating about a nominal coil inductance, at a frequencyslightly above f_(n), the impedance of the LC current becomes highlysensitive to the magnetic air gap 22. As the gap opening g decreases,the inductance increases since L is proportional to 1/g. The resonantfrequency decreases, impedance increases, and the current decreases. Theopposite occurs in the diametrically opposed coil. These currentchanges, responsive to rotor position changes, particularly whenoperating in a "push-pull" fashion with diametrically opposed controlcoils 42, allows passive radial position stabilization without radialposition sensors and without the attendant controller electronics, atleast for certain conditions. Limitations of this approach includeincreased power losses in the chamber wall due to the eddy currents setup by the AC excitation, interference with the AC motoring excitement ofthe drive coil 40, limited flexibility in tailoring the system dynamics,and reduced damping capabilities.

Another alternative to radial position control, one which eliminates thesensors, but not the controller, is position estimation. One methodsuperposes a small signal as a high carrier frequency on the controlcoil current. This high frequency signal provides a measure of theimpedance (as discussed above with respect to the LC circuit), which inturn is a measure of the air gap in the magnetic circuit, and hence ofthe position of the rotor. A second method directly measures the currentin the control coils 42. The sensed current value, as noted above, is afunction of the inductive impedance of the associated coil 42, which inturn is a function of the magnitude of the air gap 22. A micropressor inthe controller is programmed to translate the sensed current changesinto rotor position changes. The controller then uses this positioninformation in the usual way to make the appropriate corrections in thecontrol coil currents to drive the rotor to a pre-selected position andorientation. As used herein, "sensing" is mean to include positionsensing using transducers as well as the LC and estimation approachesdescribed herein and other techniques known to those skilled in the art.As noted above, ideally position control is totally passive to avoid thecost of sensors and active control electronics. Total passive controlworks well for sufficiently low rotational speeds, but for operationhigh speeds, e.g. 1200 rpm, some degree of active control is preferred.

While the invention has been described with respect to a homopolarmotor, it is also possible to use the IMMSS 10 with an induction motor.This allows the system to operate off a fixed frequency and excitationvoltage. It also eliminates the proximity sensors 52 and electroniccommutators for the phase currents of the homopolar motor. Further, aninduction motor avoids the torque fluctuations produced by the poles ofthe homopolar motor. The IMMSS 10 is able to operate as an asynchronousinduction motor simply by removing the rotor teeth 18c, as shown inphantom in FIG. 4. The problem of an induction drive is the strong ACfield necessary to spin up to normal operation at the fixed frequency.

With the homopolar motor, different poles shapes and counts can be used.Two to eight pole pairs are preferred. Higher pole counts reduce theamount of steel required in the rotor and stator, but increases in therequired excitation frequency and the magnetizing current are required.End turn volume and chamber wall losses are also design considerations.

There has been described an integrated magnetic motoring and suspensionsystem which can be used in semiconductor wafer processing to rotate thewafer in a chamber with a controlled and hostile atmosphere withoutmoving contact which itself generates particle contaminants. Theintegration lends itself to a high degree of compactness which isnecessary for a commercially practical IMMSS. The IMMSS of the inventionreduces wafer production down-time due to bearing failure and to cleanup associated with wafer breaks and bearing failure. It is also highlycompatible with robotics that can provide fully integrated, contact-lesswafer fabrication. It also allows the processing of larger diameter(e.g. >300 mm) wafers and at higher rotational speeds (e.g. 1200 rpm)than heretofore possible with conventional equipment using mechanicalbearings. It allows precise positioning of the wafer not impaired byfriction and stiction. It has a long operational life as compared toconventional mechanical bearing wafer processing systems. It produces nomechanical vibration and isolates the wafer from external vibration andshock. It also provides excellent in-chamber geometry--the region aboveand below the wafer is unobstructed, there are few internal components,and those few components are readily removed and cleaned. The systemdescribed herein is also highly flexible. It can handle differentrotated bodies (wafer, non-wafers, wafers of varying sizes), differentoperating environments, a range of rotational speeds, and a wide rangeof other cost/performance trade-offs.

While the invention has been described with respect to its preferredembodiments, it will be understood that various alterations andmodifications will occur to those skilled in the art from the foregoingdetailed description and the accompanying drawings. For example, whilethe invention has been described with respect to permanent magnets 38aestablishing the DC flux field, it is possible to use an iron-core coilor coils, or other equivalents such as combination of coils and magneticmaterials. As used herein, "permanent magnet" includes all sources of DCmagnetic flux. Likewise, while plate-type pole pieces have beendescribed with four pole segments, this particular geometry and polecount is not essential. What is essential is to have poles which carrythe flux across a small air gap to the rotor so that the fluxsimultaneously performs the levitation, radial biasing and rotor fluxfield magnetization functions described above. Similarly, while thefield coils and control coils are described as end-wound andcentrally-wound, respectively, other locations for these windings arepossible. However, to modulate the DC flux, the control coils should bedirectly coupled with the DC magnetic circuit. The stator and rotor canalso be laminated to control eddy current losses, but with attendantincreases in cost. Likewise, the rotor can be formed with a differentnumber of poles, single salient pole teeth, or, as described above withrespect to an asynchronous inductive motor, with no pole teeth. Theseand other modifications and variations occurring to those skilled in theart are intended to fall within the scope of the appended claims.

What is claimed is:
 1. A method for levitating a mass and rotating themass about a vertical z-axis with a motor having a stator assembly and arotor, the rotating occurring without frictional contact between thestator assembly and the rotor, which have an air gap therebetween, saidmethod comprising:forming the rotor as a ring of ferromagnetic materialhaving a high magnetic permeability, said ring comprising a verticalportion extending along the z-axis and a horizontal portion extendingperpendicular to the z-axis, levitating and centering the rotor withrespect to the stator assembly along the z-axis with a stator magneticflux field comprising a DC flux field, which also centers the rotoragainst tilt out of a plane orthogonal to the z-axis, said DC flux fieldoriginating at the stator assembly, simultaneously radially biasing therotor with said stator magnetic flux field, simultaneously inducing aflux field in said rotor with said stator magnetic flux field appliedacross an air gap through said horizontal portion of the ring, drivingthe rotor to rotate about the z-axis using a rotating electromagneticfield produced by the stator assembly and applied across the air gap,the rotating electromagnetic field interacting with said flux fieldinduced in the rotor by said stator magnetic flux field, sensing atleast a radial position of the rotor with respect to the statorassembly, and adjusting the stator magnetic flux at angularly spacedregions about the stator assembly in response to said sensing tomaintain a spacing between said rotor and said stator assembly duringsaid driving.
 2. The method of claim 1, wherein said rotor is acontinuous ring of a ferromagnetic material and comprises a portionhaving a C-shape cross section.
 3. The method of claim 1, wherein saidstep of driving the rotor is synchronous and homopolar.
 4. The method ofclaim 1, wherein said driving is asynchronous and inductive.
 5. Themethod of claim 1, wherein said adjusting step comprises providing coilsat each of said regions and altering an excitation current in the coils.6. The method of claim 1, wherein said adjusting step comprises tuningan AC excited LC circuit at each of said regions where the inductance Lis that of a coil in one of said angularly spaced regions whose magneticfield produces said adjusting without an aid of discrete positionsensors.
 7. The method of claim 1, wherein said adjusting step comprisesproviding a dynamic estimation of the rotor-to-stator assembly spacingwithout an aid of discrete position sensors.
 8. An integrated magneticmotor and suspension device that rotates a mass about a vertical z-axisthat is mutually orthogonal with x and y axes, said device comprising:aferromagnetic rotor that lies generally in an x-y plane defined by saidx and y axes and is rotatable about the z-axis, said rotor comprising avertical portion extending along the z-axis and a horizontal portionextending perpendicular to the z-axis, a stator assembly that extendsgenerally in said x-y plane and is closely spaced from the rotor by anair gap, said stator assembly having an even number of stator polesegments, each of said stator pole segments comprising:(i) upper andlower pole pieces, (ii) a permanent magnet coupled between said upperand lower pole pieces to produce a DC magnetic flux field across the airgap, said DC magnetic flux (a) centers the rotor along the z-axis and inan orientation generally orthogonal to the z-axis with a passivereluctance, (b) provides a radial bias force on the rotor, and (c)induces a DC flux field at said rotor, (iii) windings mounted on saidpole pieces to drive the rotor across the air gap to rotate about thez-axis through an interaction with said DC flux field at said rotor, and(iv) a pair of position control coils, one of said coils located on anupper pole plate and an another one of said coils on a lower pole plateof a stator pole segment between the permanent magnet and therotor,wherein a plurality of pairs of position control coils areequiangularly spaced around said stator assembly to produce a magneticflux that adds and subtracts from the flux field of said DC permanentmagnetic field to radially center said rotor with respect to said statorassembly and to supplement a passive tilt control provided by saidpermanent magnet, means for sensing at least an actual radial positionof the rotor and producing an electrical signal indicative of a sensedradial position, and means for producing position control currents ineach of said plurality of position control coils in response to saidelectrical signal.
 9. The integrated magnetic motor and suspensionsystem of claim 8, wherein said rotor is a continuous ring with pluralpole teeth formed at its periphery and wherein said pole pieces havecorresponding poles formed on their rotor-facing peripheries to producea synchronous homopolar drive.
 10. The integrated magnetic motor andsuspension system of claim 8, wherein said rotor is a continuous ringwith no pole teeth and said stator assembly windings produce anasynchronous inductive drive for said rotor.
 11. The integrated magneticmotor and suspension system in claim 8, wherein said coils are activecoils disposed in diametrically opposed arrays on said pole pieces. 12.The integrated magnetic motor and suspension system of claim 8, whereinsaid coils are each connected in series with a capacitor and excited byan AC current at a frequency such that the inductance of the associatedcoil provides a passive radial position control over said rotor.
 13. Theintegrated magnetic motor and suspension system of claim 8, wherein saidposition control current producing means includes means for sensing themagnitude of said currents and calculating from said sensed magnitudesthe radial position of the rotor with respect to the stator assembly.14. The integrated magnetic motor and suspension system of claim 8,wherein said sensing means comprises a plurality of radial and verticalrotor position sensors.
 15. An integrated magnetic support androtational drive for processing semiconductor wafers in a sealablechamber, comprising,a rotor with plural poles formed of a ferromagneticmaterial located in the chamber and adapted to support the wafer in thechamber and rotate it about a vertical z-axis while it extends in an x-yplane orthogonal to said vertical z-axis, said rotor comprising avertical portion extending along the z-axis and a horizontal portionextending perpendicular to the z-axis, a stator located in a closelyspaced relationship to said rotor with an air gap therebetween andhaving,(i) a permanent magnet whose DC flux levitates said rotor,radially biases the position of said rotor, and induces a DC flux fieldin said rotor across the air gap, and (ii) upper and lower pole platesthat sandwich said permanent magnet and are each divided into pluralequiangularly-spaced pole segments,(a) a polyphase winding wound on saidpole plates that produces a rotating electromagnetic field thatinteracts with said induced DC flux in said rotor to rotate said rotorand the wafer supported on said rotor about said z-axis in said x-yplane, and (b) a position control coil wound on each of said pole platesat each stator plate segment, rotor position sensing means thatdetermine at least the radial and vertical positions of said rotor, anda feedback control circuit responsive to said position sensing meansthat said position control coils to center said rotor radially andagainst tilt out of said x-y plane by modulating the DC flux of saidpermanent magnet acting on said rotor.